(1) Field of the Invention
The present invention relates lies in the field of power plants, and more particularly heat exchanger devices for use in power plants. The present invention relates in particular to a power plant having a two-stage cooler device for cooling the admission air for at least one fuel-burning engine of the power plant. The present invention also relates to a two-stage method of cooling the admission air for at least one fuel-burning engine of such a power plant.
The power plant is intended more particularly for a rotary wing aircraft and it serves to cool the air admitted into at least one gas turbine of the power plant between two compression stages for compressing the admission air, upstream from a combustion chamber in each gas turbine.
(2) Description of Related Art
It is known that increasing the pressure of admission air prior to injecting it into the combustion chamber of a fuel-burning engine increases the efficiency of the engine and also the power that it can deliver. In contrast, compressing admission air leads to an increase in its temperature, and consequently to a decrease in its density. Cooling the admission air after it has been compressed serves once more to improve the effectiveness of the heat engine and to increase its efficiency and the power it delivers.
For example, with road vehicle engines, an air-air heat exchanger is often used for cooling the admission air leaving a turbocharger prior to entering into the combustion chamber of the engine.
Likewise, in known manner in the field of turboshaft engines, an air-air heat exchanger is used to cool the admission air leaving an intermediate stage of a compressor prior to being reinjected into the following stage of the compressor, upstream from the combustion chamber of the engine.
In both of those examples, cooling the admission air serves at the end of the compression stage to procure admission air at a lower temperature, thereby enabling the power delivered by the engine to be increased. That type of heat exchanger for cooling air is generally referred to as an “intercooler”. Such an intercooler is often an air-air heat exchanger, but it could also be an air-liquid heat exchanger.
Document U.S. Pat. No. 8,813,503 is also known, which describes a method and a system for managing the temperature to which admission air for a turboshaft engine is cooled in order to limit condensation from the admission air on passing through an intercooler situated between two compression stages of a gas turbine. That system serves in particular to control the temperature of the air in a cooler for cooling the admission air that is situated upstream from the two compression stages and the intercooler.
Unfortunately, using such an intercooler on board a rotary wing aircraft for the purpose of increasing the power delivered by the turboshaft engine(s) of the aircraft is difficult, and as a result is not used at present.
Firstly, incorporating an intercooler, which is usually an air-air heat exchanger, in the vicinity of the turboshaft engine of an aircraft, and in particular in the proximity of the zone containing its compressors, is difficult. The dimensions of the intercooler can be large if the intercooler is to be capable of achieving an advantageous increase in power from the engine. Such dimensions are then unfavorable for installing the intercooler in an aircraft.
Furthermore, the weight of the intercooler can also be considerable and the ratio of the resulting power increase to increase in aircraft weight is small and possibly close to zero.
As a result, the dimensions of the intercooler need to be limited in order to enable an intercooler to be installed in a rotary wing aircraft. However, the increase in the power from the engine of the aircraft is then small and the advantage of such an installation becomes limited.
Finally, when installing an intercooler in a rotary wing aircraft, it is often found to be complex to convey cooling air to the intercooler, and that can limit its effectiveness, and consequently the improvement in power obtained from the engine of the aircraft.
Furthermore, an intercooler, and in general manner a heat exchanger, constitutes a heat engine that uses only one source of heat. The source of heat is generally ambient air in an air/air heat exchanger. Such a heat engine can be referred to as an engine having a single heat source. Such single heat source heat engines are limited to exchanging heat between two fluids.
There also exist heat engines that use a plurality of heat sources. Such heat engines are capable, when they provide drive, of converting heat energy into mechanical energy, or else, when they receive drive, of converting mechanical energy into heat energy. Such heat engines use a fluid that is subjected to cyclical transformations during which the fluid exchanges energy with the outside in the form of work, and exchanges energy with the heat sources in the form of heat.
Heat engines having two heat sources, i.e. using two heat sources at different temperatures, are also known, such as for example a spark ignition engine, a steam power station, or a refrigerator machine.
Heat engines are also known that make use of three heat sources. Such heat engines using three heat sources are used in particular as refrigerator machines using a known ejector refrigerating cycle.
Such an ejector refrigerating cycle can be summarized as follows:
at the outlet from a condenser, a refrigerant fluid in liquid form is directed firstly to a drive loop and secondly to a refrigerating loop;
the drive loop includes a pump that compresses a first portion of the refrigerant fluid and a first evaporator in which the first portion of the refrigerant fluid is transformed into gaseous form;
the refrigerating loop includes an expander that expands a second portion of the refrigerant fluid and a second evaporator in which the second portion of the refrigerant fluid is transformed into gaseous form; and                thereafter the first portion of the refrigerant fluid is used as driving refrigerant fluid in an ejector serving firstly to compress and drive the second portion of refrigerant fluid and secondly to mix together the two portions of refrigerant fluid prior to them entering the condenser so as to transform the refrigerant fluid into liquid form, the cycle thus being looped.        
The three heat sources are used respectively in the two evaporators and in the condenser for exchanging heat energy with the refrigerant fluid.
Such heat engines, whether they have two or three heat sources, make use of thermodynamic cycles such as the “Carnot” cycle or the “Rankine” cycle.
The Rankine cycle is a thermodynamic cycle that is similar to the Carnot cycle. It differs therefrom by replacing two constant-temperature (isothermal) transformations in the Carnot cycle with two constant-pressure (isobaric) transformations. The cycle is thus made up of four successive transformations: adiabatic compression; constant-pressure vaporization; adiabatic expansion; and constant-pressure liquefaction.
Industrial applications of the Rankine cycle include for example systems making use of the heat that is lost by industrial processes in order to provide additional electrical power supply. The Rankine cycle is used in particular in steam power stations, including nuclear power stations.
The Rankine cycle is also used with organic fluids having a vaporization temperature lower than that of water. The temperatures of the heat sources used with such a Rankine cycle can thus be low.
By way of example, Document U.S. Pat. No. 8,438,849 describes a heat recovery system using two heat sources comprising a high pressure turbine and a low pressure turbine. Those two turbines operate using the Rankine cycle and they serve to generate mechanical energy that is then transformed into electricity.
Also known is Document US 2010/0242479, which describes a system for recovering energy by using at least two heat sources at different temperatures and a plurality of Rankine cycles in cascade. That energy recovery system serves to generate both mechanical energy that can be transformed into electricity, and also heat energy for the purpose of cooling and/or heating an additional fluid via one or more heat exchangers.
Furthermore, an absorber machine can also be used for cooling the admission air for a fuel-burning engine between two compression stages of the admission air, as described in Document EP 2 295 765. That absorber machine includes in particular two evaporators, two condensers, a pump, and two expanders. That absorber machine uses ammonia (NH3) or else lithium bromide (LiBr) which have the disadvantage of being fluids that are highly toxic. Nevertheless, such an absorber machine is generally of large dimensions, so its overall size and weight are then incompatible with the constraints required for being incorporated in an aircraft. Furthermore, the operation of an absorber machine requires a set-up that is stationary, in particular for absorption of the refrigerant by the absorbent. This requirement is prohibitive for integrating in a moving vehicle.
Finally, Document U.S. Pat. No. 4,490,989 describes a system for heating and air conditioning an aircraft cabin using a heat engine comprising an evaporator, a condenser, and a compressor, through which a refrigerant fluid circulates. The cabin of the aircraft may be fed with cooled air leaving the evaporator or with air from outside the aircraft heated by the exhaust gas of a fuel-burning engine of the aircraft.